Direct current (DC) power transmission systems are used today to interconnect alternating current (AC) power systems and to transmit power at high voltages over long distances. These systems are known in the art as High Voltage Direct Current (HVDC) transmission systems. The main parts of a HVDC system are the DC link in form of one or a multiple of power cables or overhead lines and a converter station at each end of the DC link containing a power converter. The power converter which transforms AC into DC is called rectifier, and the power converter transforming DC into AC is called inverter. The rectifier transfers active power from the AC side to the DC side and the inverter transfers active power from the DC side to the AC side. Hence, the power in the DC link flows from the rectifier to the inverter.
The HVDC converters which are subject of the present invention are voltage source converters (VSC). The functional principle of such a converter type is widely known in the art, see for example Anders Lindberg, “PWM and Control of Two and Three Level High Power Voltage Source Converters”, KTH Stockholm 1995, ISSN-1100-1615, in particular pages 1, 77-104 and appendix A.
A HVDC system can for example be used to link two independent AC power grids so that power can be transferred from one grid to the other at varying rates according to power trade purposes. A HVDC system may also be used to connect a power generation plant to an AC grid, where the power generation plant provides power and voltage with varying quality and stability and where the possibility to control the VSCs of the HVDC system is used to compensate for these variations in order to fulfil the requirements of power infeed into an AC grid. An example for such an application is the connection of a wind park or wind farm to an AC grid, where the wind farm could be an offshore-installation. Nowadays, large wind farms can be found which comprise wind mills with more than 2.0 Megawatt output power. In wind mills of that size it is common practice to use so called adjustable speed generators (ASG) to transform the wind energy into electrical power. ASGs are cost effective and provide a simple pitch control of the propeller at reduced mechanical stress. The ASGs available on the market comprise either a synchronous generator and two full-sized converters which connect the stator of the synchronous generator to the AC line or AC bus of the wind farm or a doubly-fed induction generator, where the rotor is coupled via two back-to-back connected VSCs and the stator is connected directly to the AC line or AC bus of the wind farm. The AC line or AC bus of the wind farm is called local AC bus in the following. As already described, the local AC bus is itself connected via a HVDC system to an AC power grid.
ASGs have the advantage that due to the control of the VSCs the wind mill delivers power at constant frequency to the local AC bus. This implies at the same time that the power generation by the synchronous or induction generator in the ASG is working independently of frequency or phase angle variations of the AC voltage in the local AC bus. Opposed to that, a synchronous or induction generator connected directly to an AC line would react to a sudden increase of the frequency of the AC voltage with a reduction in the generated power. However, this is not the case for an ASG.
One or a multiple of ASGs connected via a HVDC system with VSCs to an AC grid may encounter the following problem. If an AC fault occurs in the AC grid so that only a reduced or zero power can be fed into the grid, the DC voltage in the DC link will increase due to the fact that the VSC connected to the wind farm keeps delivering the defined constant power from the AC to the DC side. The increase of the DC voltage will happen very quickly since the capacitance of the DC link has a comparatively small time constant.
To overcome this problem it is known in the art to use a DC chopper, which is composed of at least one switchable resistor. In case of a fault in the AC grid which leads to a reduced power infeed into the grid, the at least one resistor is switched to be connected in parallel to the DC link, so that overpower delivered by the wind farm is absorbed in the resistor. The power rating for the chopper resistor or resistors needs to be extraordinarily high, since power of up to a few hundreds of MW is transmitted from wind farms over HVDC links in today's applications. Apart from that it is common to use an IGBT as switch in order to achieve fast control. The known DC chopper installations require considerable space and are costly. In addition, the DC chopper control function causes disturbances in the usual HVDC control which make it difficult to achieve a smooth and stable recovery of the system after the fault in the AC grid is cleared. A power regulation by control of the VSC in the HVDC system which is connected between the wind farm and the DC link, where this VSC is referred to as first voltage source converter in the following, is regarded in the art as non-effective since the power generation of ASGs can not be affected by AC frequency variations, as explained above.